Method and apparatus for detecting a synchronous signal

ABSTRACT

A method for detecting a synchronous signal contained in an input image signal is provided. In the method, horizontal and vertical periods are established during which horizontal and vertical synchronous signal pulses contained in the input image signal are counted, respectively. Then, minimum and maximum horizontal values which correspond to the horizontal synchronous signal pulses contained in the horizontal period are established, and minimum and maximum vertical values which correspond the vertical synchronous signal pulses contained in the vertical period are established. Afterwards, the horizontal and vertical synchronous signal pulses are counted during the horizontal and vertical periods to respectively obtain first and second counted values. Then, the method determines if the first counted value is greater than or equal to the minimum horizontal value and less than or equal to the maximum horizontal value and determines if the second counted value is greater than or equal to the minimum vertical value and less than or equal to the maximum vertical value. Subsequently, the method determines than an actual synchronous signal is contained in the input image signal if the first counted value is greater than or equal to the minimum horizontal value and less than or equal to the maximum horizontal value and the second counted value is greater than or equal to the minimum vertical value and less than or equal to the maximum vertical value.

FIELD OF THE INVENTION

The present invention relates to an image signal processing method and acorresponding apparatus. More particularly, the present inventionrelates to a synchronous signal determining method and apparatus whichcan accurately detect a normal image signal despite interference causedby noise.

BACKGROUND OF THE INVENTION

In general, a television display of an NTSC broadcasting system forms animage frame with two field signals, and one frame is generated from 525line signals which are contained in an image signal. The image signalalso comprises a vertical synchronous signal V-SYNC for synchronizingthe two field signals with each other and contains a horizontalsynchronous signal H-SYNC for synchronizing the 525 line signals witheach other.

In an image signal recording/reproducing apparatus, the synchronoussignals V-SYNC and H-SYNC are detected and utilized for various signalprocessing operations. For example, if a synchronous signal is notdetected from an image signal that is input when a television isswitched from a broadcasting channel to another broadcasting channel,the apparatus determines that no image signal exists. In such instance,a noise picture, which is normally displayed when a synchronous signalis not detected, is replaced by a blue background picture or anotherreference picture. Thus, tuning to a channel which is not carrying animage signal is more aesthetically pleasing to a viewer.

In a conventional method for detecting an image signal, the input of animage signal is detected by counting the number of synchronous signalsgenerated during one field or one frame of the image signal or bydetermining the cycle of the synchronous signal. However, various noisesexist in an image signal recording/reproducing apparatus. Thus, when thefrequency (or pulse width) of the noises are similar to the frequency(or pulse width) of the synchronous signals, the noises can be misjudgedas synchronous signals. (See FIG. 7A). As a result, the methodincorrectly detects the presence of an image signal. Similarly, if noisesignificantly interferes with a valid synchronous signal, the method mayincorrectly detect the absence of an image signal.

SUMMARY OF THE INVENTION

In order to overcome the above problems, an object of the presentinvention is to provide a method and apparatus which correctly detectthe input of a synchronous signal without error in spite of noise whichis similar to the synchronous signal.

In order to accomplish the above and other objects, a method fordetermining if a synchronous signal is contained in an input imagesignal is provided. The method comprises the steps of: (a) establishinga horizontal period during which horizontal synchronous signal pulsescontained in said input image signal are counted; (b) establishing avertical period during which vertical synchronous signal pulsescontained in said input image signal are counted; (c) establishing aminimum horizontal value and a maximum horizontal value which correspondto a proper number of said horizontal synchronous signal pulsescontained in said horizontal period; (d) establishing a minimum verticalvalue and a maximum vertical value which correspond to a proper numberof said vertical synchronous signal pulses contained in said verticalperiod; (e) counting a first number of said horizontal synchronoussignal pulses during said horizontal period to obtain a first countedvalue; (f) counting a second number of said vertical synchronous signalpulses during said vertical period to obtain a second counted value; and(g) determining if said first counted value is greater than or equal tosaid minimum horizontal value and less than or equal to said maximumhorizontal value; (h) determining if said second counted value isgreater than or equal to said minimum vertical value and less than orequal to said maximum vertical value; and (i) determining than an actualsynchronous signal is contained in said input image signal if said firstcounted value is greater than or equal to said minimum horizontal valueand less than or equal to said maximum horizontal value and said secondcounted value is greater than or equal to said minimum vertical valueand less than or equal to said maximum vertical value.

In order to further accomplish the above and other objects, a method fordetermining if a synchronous signal is contained in an input imagesignal is provided. The method comprises the steps of: (a) establishinga vertical period during which vertical synchronous signal pulsescontained in said input image signal are counted; (b) establishing aminimum vertical value and a maximum vertical value which correspond toa proper number of said vertical synchronous signal pulses contained insaid vertical period; (c) establishing a minimum noise count value and amaximum noise count value which correspond to a number of noise pulsesthat are estimated to be similar to said vertical synchronous signalpulses in a noise gate pulse period; (d) generating a noise gate pulsebetween adjacent pulses of said vertical synchronous signal pulses; (e)counting a first number of said vertical synchronous signal pulsesduring said vertical period to obtain a first counted value; (f)determining if said first counted value is greater than or equal to saidminimum vertical value and less than or equal to said maximum verticalvalue; and (g) generating a negative determination signal as a currentdetermination signal when said first counted value is less than or equalto said minimum vertical value or is greater than or equal to saidmaximum vertical value, wherein said negative determination signalindicates that an actual synchronous signal is not contained in saidinput image signal; (h) counting a second number of said noise pulsesduring said noise gate pulse period to obtain a second counted value;and (i) generating a positive determination signal as said currentdetermination signal when said first counted value is greater than orequal to said minimum vertical value and less than or equal to saidmaximum vertical value and when said second counted value is less thansaid minimum noise count value, wherein said positive determinationsignal indicates that an actual synchronous signal is contained in saidinput image signal.

In order to additionally accomplish the above and other objects, anapparatus for determining if a synchronous signal is contained in aninput image signal is provided. The apparatus comprises: a verticalsynchronous signal separator which separates vertical synchronous signalpulses from composite synchronous signals; a first counter which isoperably connected to said vertical synchronous signal separator andwhich counts a first number of said vertical synchronous signal pulsesthat exist during a first predetermined period to obtain a first countedvalue; a first comparator which is operably connected to said firstcounter and which generates a first detection signal when said firstcounted value is greater than or equal to a predetermined minimumvertical value and is less than or equal to a predetermined maximumvertical value; a second counter which inputs horizontal synchronoussignal pulses contained in said composite synchronous signals and whichcounts a second number of said horizontal synchronous pulses during asecond predetermined period to generate a second counted value; a secondcomparator which is operably connected to said second counter and whichgenerates a second detection signal when said second counted value isgreater than or equal to a predetermined minimum horizontal value and isless than or equal to a predetermined maximum horizontal value; and anoperator circuit which inputs said first detection signal and saidsecond detection signal and generates a determining signal based on saidfirst detection signal and said second detection signal.

In order to even further accomplish the above and other objects, anapparatus for determining if a synchronous signal is contained in aninput image signal is provided. The apparatus comprises: a verticalsynchronous signal separator which separates vertical synchronous signalpulses from composite synchronous signals contained in said input imagesignal; a first counter which is operably connected to said verticalsynchronous signal separator and which counts a first number of saidvertical synchronous signal pulses that exist during a firstpredetermined period to obtain a first counted value; a noise gate pulsegenerator which inputs said vertical synchronous signal pulses andgenerates a noise gate pulse between adjacent vertical synchronoussignal pulses; a logic switch portion which is operably connected tosaid noise gate pulse generator and which inputs said compositesynchronous signals and said noise gate pulses, processes said compositesynchronous signals and said noise gate pulses, and outputs noisepulses; a second counter which is operably connected to said logicswitch portion and which counts a second number of said noise pulses toobtain a second counted value; and a comparator which is operablyconnected to said first counter and said second counter and whichgenerates a current determining signal having a first value indicatingthat an actual synchronous signal is contained in said input imagesignal when said first counted value and second counted values haveparticular values.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and advantages of the present invention will becomemore apparent by describing in detail a preferred embodiment thereofwith reference to the attached drawings in which:

FIG. 1 is a block diagram of a synchronous signal determining deviceaccording to an embodiment of the present invention;

FIG. 2 is a block diagram of a synchronous signal determining deviceaccording to another embodiment of the present invention;

FIG. 3 is a flow chart of a synchronous signal determining methodaccording to an embodiment of the present invention;

FIG. 4 is a flow chart of a synchronous signal determining methodaccording to another embodiment of the present invention;

FIG. 5 is an example of a detailed circuit diagram of a verticalsynchronous signal separator shown in FIGS. 1 and 2;

FIG. 6A is an example of a waveform of a composite synchronous signalinput to the circuit shown in FIG. 5;

FIG. 6B is an example of a waveform of a vertical synchronous signaloutput from the circuit shown in FIG. 5;

FIG. 7A is an example of a waveform of noise which may be misinterpretedas vertical and horizontal synchronous signals;

FIG. 7B is an example of a waveform of a composite synchronous signal;

FIG. 7C is an example of a waveform of a vertical synchronous signal;

FIG. 7D is an example of a waveform of a horizontal synchronous signal;

FIG. 7E is an example of a waveform of a first masking pulse output fromthe first noise masking portion shown in FIG. 1;

FIG. 7F is an example of a waveform of a second masking pulse outputfrom the second noise masking portion shown in FIG. 1; and

FIG. 7G is an example of a waveform of a noise gate pulse output fromthe noise gate pulse generator shown in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments disclosesspecific circuit configurations, components, values, etc. However, thepreferred embodiments are merely examples of the present invention, andthus, the specific components and values described below are only usedto more easily describe such embodiments and to provide an overallunderstanding of the present invention. Accordingly, one skilled in theart will readily recognize that the present invention is not limited tothe specific components and values described below. Furthermore, thedescriptions of various features and structures of the present inventionwhich would have been known to one skilled in the art are omitted forthe sake of clarity and brevity.

FIG. 3 illustrates an example of a method for detecting a synchronoussignal according to an embodiment of the present invention. In themethod, initial values are set for first and second periods T1 and T2,minimum and maximum horizontal values N1 and N2, and minimum and maximumvertical values M1 and M2 (step 310).

In an NTSC broadcasting system, the first period T1 coincides with atime during which pulses of a horizontal synchronous signal H-SYNC isinput. Also, the period T1 should preferably be set to a time which islarger than a minimum cycle of a horizontal synchronous signal H-SYNC sothat the number of synchronous signals H-SYNC can be adequately counted.As shown in FIG. 7D, the period T_(HS) of the horizontal synchronoussignal H-SYNC is 64 μsec, and thus, the first period T1 should begreater than such period T_(HS). Similarly, the second period T2coincides with a time during which pulses of a vertical synchronoussignal V-SYNC is input. Also, the period T2 should preferably be set toa time which is larger than a minimum cycle of a vertical synchronoussignal V-SYNC so that the number of synchronous signals V-SYNC can beadequately counted. As shown in FIG. 7C, the period T_(VS) of thevertical synchronous signal V-SYNC is 16.67 msec (in an NTSCbroadcasting system), and thus, the second period T2 should be greaterthan such period T_(VS). The values N1 and N2 respectively relate to theminimum and maximum number of pulses of the horizontal synchronoussignals H-SYNC which should occur during the first period T1. Suchvalues N1 and N2 should be set by considering the fact that some of thehorizontal synchronous signals H-SYNC may be overlooked due to noise orthat noise may be misidentified as synchronous signals H-SYNC. Likewise,the values M1 and M2 respectively relate to the minimum and maximumnumber of pulses of the vertical synchronous signals V-SYNC which shouldoccur during the second period T2. Also, the values M1 and M2 shouldbased on the fact that a portion of the vertical synchronous signalsV-SYNC may be overlooked due to noise or that noise may be misidentifiedas synchronous signals V-SYNC.

Then, the pulses which are input during the first period T1 and whichhave a particular frequency (or pulse width) value are detected ashorizontal synchronous signals H-SYNC, and the number of detectedhorizontal synchronous signals H-SYNC are counted to produce a firstcounted value N (step 320). Not only are actual horizontal synchronoussignals H-SYNC counted during the period T1, but any noise which ismisidentified as a horizontal synchronous signal H-SYNC is also counted.Also, a pulse of an actual horizontal synchronous signal H-SYNC which isaffected by noise may not be counted.

Afterwards, the pulses which are input during the second period T2 andwhich have a particular frequency (or pulse width) value are detected asvertical synchronous signals V-SYNC, and the number of detected verticalsynchronous signals V-SYNC are counted to produce a second counted valueM (step 330). As in the case of the detected horizontal synchronoussignals H-SYNC, not only are actual vertical synchronous signals V-SYNCcounted during the period T2, but any noise which is misidentified as avertical synchronous signal V-SYNC is also counted. Also, a pulse of anactual vertical synchronous signal V-SYNC which is affected by noise maynot be counted.

After the counted values M and N have been calculated, the methoddetermines if the first counted value N is between the minimum andmaximum horizontal values N1 and N2 (step 340). If the first countedvalue N is not within such range, the method determines that an actualhorizontal synchronous signal H-SYNC has not been detected within thetime period T1. Thus, the method determines that a synchronous signal isnot being input, and a non-detection signal is generated (step 370).

On the other hand, if the first counted value N is between the minimumand maximum horizontal values N1 and N2, the method determines that anactual horizontal synchronous signal H-SYNC has been detected. Then, themethod determines if the second counted value M is between the minimumand maximum vertical values M1 and M2 (step 350). If the second countedvalue M is smaller than the minimum vertical value M1 or larger than themaximum vertical value M2, the method determines that an actual verticalsynchronous signal V-SYNC has not been detected within the time periodT2. Thus, the method determines that a synchronous signal is not beinginput, and the non-detection signal is generated (step 370). On theother hand, if the second counted value M is between the minimum andmaximum vertical values M1 and M2, the method determines that an actualvertical synchronous signal V-SYNC has been detected. Thus, the methoddetermines that a synchronous signal is being input, and a detectionsignal is generated (step 360).

As shown above, the method determines that a synchronous signal is inputonly when a proper number of horizontal synchronous signals H-SYNC aredetected during the time period T1 and a proper number of verticalsynchronous signals V-SYNC are detected during the time period T2. If anincorrect number of horizontal synchronous signals H-SYNC or verticalsynchronous signals V-SYNC are detected, the method determines that asynchronous signal is input. Also, the detection of vertical andhorizontal synchronous signals V-SYNC and H-SYNC is made whileconsidering the influence of noise on the synchronous signals V-SYNC andH-SYNC. Therefore, the present embodiment can more accurately determinewhether or not a synchronous signal is currently being input.

FIG. 4 illustrates another embodiment of a method for detecting asynchronous signal. In the method, initial values are set for a periodTv, minimum and maximum values Mv1 and Mv2, and minimum and maximumnoise values NC1 and NC2 (step 410). The period Tv coincides with a timeduring which pulses of vertical synchronous signals V-SYNC are input andis preferably set to a time which is larger than a minimum cycle of thesignals V-SYNC. The maximum and minimum values Mv1 and Mv2 respectivelyrelate to the minimum and maximum number of pulses of signals V-SYNCwhich should occur during the period Tv. Also, the values NC1 and NC2correspond of the minimum and maximum number of noise pulses which canbe recognized as vertical synchronous signals V-SYNC during a periodwhen a noise gate pulse is generated.

In a preferred embodiment, the period Tv should be set to be longer thanthe time corresponding to the minimum cycle of a vertical synchronoussignal V-SYNC for both NTSC and PAL broadcasting systems in order toproperly count the number of pulses of the vertical synchronous signalsV-SYNC. Also, the values Mv1 and Mv2 should be set by considering thefact that a portion of the vertical synchronous signals V-SYNC may beoverlooked due to noise or that noise may be misidentified as thesynchronous signals V-SYNC.

After the various values are set in step 410, the noise gate pulses fordetecting noise are generated between adjacent pulses of the verticalsynchronous signals V-SYNC (step 420). The noise gate pulses aretriggered by the vertical synchronous signals V-SYNC as shown in FIG.7G.

Then, the pulses which are input during the period Tv and which have aparticular frequency (or pulse width) value are detected as verticalsynchronous signals V-SYNC, and the number of detected verticalsynchronous signals V-SYNC are counted to produce a counted value Mv(step 430). Not only are actual vertical synchronous signals V-SYNCcounted during the period Tv, but any noise which is misidentified as avertical synchronous signal V-SYNC is also counted. Also, a pulse of anactual vertical synchronous signal V-SYNC which is affected by noise maynot be counted.

Afterwards, the method determines if the counted value Mv is between theminimum and maximum values Mv1 and Mv2 (step 440). If the counted valueMv is not within such range, the method determines that a verticalsynchronous signal V-SYNC is not being input. As a result, a negativedetermination signal indicating that a synchronous signal has not beendetected is generated (step 450). On the other hand, when the countedvalue Mv is larger than the minimum value Mv1 and smaller than themaximum value Mv2, the pulses of noise which have a frequency (or pulsewidth) that coincides with the vertical synchronous signals V-SYNC andwhich are input when the noise gate pulses are generated within theperiod Tv are counted to obtain a noise count value Nv (step 460).

Then, the noise count value Nv is compared with the minimum noise countvalue NC1 (step 470). When the noise count value Nv is less than theminimum noise count value NC1, the influence of noise upon thesynchronous signals can be disregarded when the synchronous signals aredetected. As a result, a positive determination signal indicating that asynchronous signal has been detected is generated (step 480).

On the other hand, when the noise count value Nv is larger than theminimum value NC1, the value Nv is compared to the maximum value NC2(step 490). If the value Nv is larger than the maximum value NC2, thenegative determination signal is generated in step 450 because themethod determines that a noise signal was detected as a synchronoussignal. When the value Nv is smaller than the maximum value NC2, thedetermination signal which was previously output is maintained (step500). In other words, if the previous determination signal is thepositive determination signal, the noise increases instantaneously. Ifthe previous determination signal is the negative determination signal,the noise decreases instantaneously. Steps 490 and 500 are performed inorder to provide a margin of hysteresis to prevent errors in a boundarynumber for the synchronous signal detection determination. In systemswhere accuracy can be lowered, the steps 490 and 500 may be omitted.

The above method for detecting a synchronous signal is performed inconsideration that noise may be mixed with image signals. Thus, themethod of the present invention can more accurately detect a synchronoussignal than conventional techniques. Also, even though the methoddetects the pulses of a vertical synchronous signal V-SYNC, the methodmay also detect pulses of a horizontal synchronous signal H-SYNC.

FIG. 1 illustrates an apparatus for detecting a synchronous signal inaccordance with one embodiment of the present invention. The apparatuscomprises a vertical synchronous signal separator 110, a first noisemasking portion 120, a controller 130, a first counter 140, a firstcomparator 150, a second noise masking portion 160, a second counter170, a second comparator 180, and a logic operation portion 190.

The vertical synchronous signal separator 110 inputs compositesynchronous signals, separates only the vertical synchronous signalsV-SYNC from the composite synchronous signals, and outputs the verticalsynchronous signals V-SYNC. An example of the detailed circuitry of theseparator 110 is shown in FIG. 5 and comprises resistors R1, R2, and R3,a capacitor C1, a diode D1, and a transistor Q1. A first terminal of theresistor R1 is connected to a input terminal, and a second terminal ofthe resistor R1 is connected to a first terminal of the capacitor C1 andto a first terminal of the diode D1. A second terminal of the diode D1is connected to a first terminal of the resistor R2 and the gate of thetransistor Q1. The second terminals of the capacitor C1 and resistor R2and the emitter of the transistor Q1 are connected to ground. The firstterminal of the resistor R3 is connected to Vcc, and the second terminalof the resistor R3 is connected to the collector of the transistor Q1and the output terminal. Based on the configuration above, when acomposite synchronous signal shown in FIG. 6A is input to the inputterminal, a vertical synchronous signal V-SYNC having a waveform shownFIG. 6B is output from the output terminal.

The first noise masking portion 120 inputs the vertical synchronoussignals V-SYNC and generates a first masking pulse between the adjacentsynchronous signals V-SYNC (FIG. 7E). The first masking pulse is used toreduce the influence of noise which is generated when the verticalsynchronous signal V-SYNC is detected and is synthesized with thevertical synchronous signal V-SYNC. As a result, noise does notsignificantly affect the detection operation of the apparatus during theperiod when the first masking pulse is generated. In the preferredembodiment, the first masking pulse is obtained by subtracting a marginvalue n1 from the cycle T_(VS) of the vertical synchronous signal V-SYNCwhile various errors are taken into account.

The controller 130 generates first and second control signals fordetermining the periods during which the number of pulses of thevertical and horizontal synchronous signals V-SYNC and H-SYNC arecalculated. Such periods respectively correspond to the periods T1 andT2 described above in conjunction with FIG. 3. The first counter 140inputs the first control signal and counts the number of pulses whichcorrespond to the vertical synchronous signals V-SYNC from among theimage signals input during the period T1. Then, the first comparator 150determines whether a first counted value N is greater than the minimumvalue N1 and less than the maximum value N2. If the value N falls withinsuch range, the apparatus generates a first detection signal. Initially,the range is determined by properly selecting experimental values forthe values N1 and N2 in consideration of the fact that the influence ofnoise is reduced by the first masking pulse.

The second noise masking portion 160 inputs the composite synchronoussignal and generates a second masking pulse between adjacent horizontalsynchronous signals H-SYNC (FIG. 7F). The second masking pulse is usedto reduce the influence of noise which is generated when the horizontalsynchronous signals H-SYNC are detected and is synthesized with thecomposite synchronous signal. As a result, noise does not significantlyaffect the detection operation of the apparatus during the period whenthe second masking pulse is generated. In the preferred embodiment, thesecond masking pulse is obtained by subtracting a margin value n2 fromthe cycle T_(HS) of the horizontal synchronous signal H-SYNC whilevarious errors are taken into account.

The second counter 170 inputs the second control signal from thecontroller 130 and counts the number of pulses which correspond to thehorizontal synchronous signals H-SYNC from among the image signals inputduring the period T2. Then, the second comparator 180 determines whethera second counted value M is greater the minimum value M1 and less thanthe maximum value M1. If the value M falls within such range, theapparatus generates a second detection signal. Initially, the range isdetermined by properly selecting experimental values for values M1 andM2 in consideration of the fact that the influence of noise is reducedby the second masking pulse.

The logic operation portion 190 inputs the first and second detectionsignals and outputs a determination signal. The determination signalindicates a synchronous signal has been detected only when both thefirst and second detection signals are generated. In the presentembodiment, the first and second detection signals are both active highsignals, and the logic operation portion 190 comprises an AND gate.

When a synchronous signal is detected by the apparatus described above,the influence of noise is reduced due to the use of the masking pulsesgenerated by the first and second noise masking portions 120 and 160.Also, the apparatus determines that a synchronous signal is input onlywhen the first and second comparators 150 and 180 detect the verticaland horizontal synchronous signals V-SYNC and H-SYNC. As a result, theapparatus detects the synchronous signal with a high accuracy despitethe presence of noise.

FIG. 2 illustrates an apparatus for detecting a synchronous signal inaccordance with another embodiment of the present invention. Theapparatus comprises a vertical synchronous signal separator 210, acontroller 220, a first counter 230, a noise gate pulse generator 240, alogic switch portion 250, a second counter 260, and a comparator 270.

The vertical synchronous signal separator 210 inputs compositesynchronous signals, separates only the vertical synchronous signalsV-SYNC from the composite synchronous signals, and outputs the verticalsynchronous signals V-SYNC. The separator 210 may be similar to theseparator 110 described above in conjunction with FIG. 1.

The controller 220 generates a control signal for determining the periodduring which the number of vertical synchronous signals V-SYNC arecalculated. Such period corresponds to the period Tv described above inconjunction with FIG. 4. The first counter 230 inputs the control signaland counts the number of pulses which correspond to the verticalsynchronous signals V-SYNC from among the image signals input during theperiod Tv to produce the counted value Mv.

The noise gate pulse generator 240 inputs the composite synchronoussignals, generates a noise gate pulse between adjacent verticalsynchronous signals V-SYNC, and outputs the noise gate pulses. The logicswitch portion 250 inputs the composite synchronous signals and thenoise gate pulses and outputs the composite synchronous signals onlywhen the noise gate pulse is generated. Accordingly, the switch portion250 outputs noise in the composite synchronous signals except for noisewhich occurs during the vertical synchronous signals V-SYNC. In apreferred embodiment, the logic switch portion 250 may comprise an ANDgate which inputs the composite synchronous signals and the noise gatepulses, performs an AND operation on such signals, and outputs the ANDedsignal. Then, the second counter 260 inputs the signals output from thelogic switch portion 250 and generates a noise count value Nv bycounting only the noise pulses which have a frequency (or pulse width)corresponding to the frequency (or pulse width) of a verticalsynchronous signal V-SYNC and which are output during the period inwhich the noise gate pulse is generated.

The comparator 270 inputs the counted value Mv from the first counter230 and inputs noise count value Nv from the second counter 260. Then,the comparator 270 subtracts the noise count value Nv from the countedvalue Mv to obtain a difference value and determines if the differencevalue is within a predetermined range. The predetermined range should beset in consideration of the influence of noise which may be generated inthe system. As noted above, the noise count value Nv is obtained bycounting the noise gate pulses, and the counted value Mv is obtained bycounting the vertical synchronous signals V-SYNC without consideringnoises. Thus, by calculating the difference value, the number of actualvertical synchronous signals V-SYNC can be detected.

If the subtraction value is within the predetermined range, theapparatus determines that an actual synchronous signal has been detectedand generates a synchronous signal detection determination signal. Onthe other hand, if the difference value is not within the predeterminedrange, the apparatus determines that an actual synchronous signal hasnot been detected and that a false synchronous signal has been detecteddue to noise. Thus, the apparatus generates a synchronous signalnon-detection determination signal.

As described above, the present invention determines if the synchronoussignal is detected by considering the influence of noise. Therefore, theinvention is able to more accurately detect synchronous signals.

The previous description of the preferred embodiments is provided toenable a person skilled in the art to make or use the present invention.Moreover, various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without the use of inventivefaculty. Therefore, the present invention is not intended to be limitedto the embodiments described herein but is to be accorded the widestscope as defined by the claims.

What is claimed is:
 1. A method for determining if a synchronous signalis contained in an input image signal, comprising the steps of: (a)establishing a horizontal period during which horizontal synchronoussignal pulses contained in said input image signal are counted; (b)establishing a vertical period during which vertical synchronous signalpulses contained in said input image signal are counted; (c)establishing a minimum horizontal value and a maximum horizontal valuewhich correspond to a proper number of said horizontal synchronoussignal pulses contained in said horizontal period; (d) establishing aminimum vertical value and a maximum vertical value which correspond toa proper number of said vertical synchronous signal pulses contained insaid vertical period; (e) counting a first number of said horizontalsynchronous signal pulses during said horizontal period to obtain afirst counted value; (f) counting a second number of said verticalsynchronous signal pulses during said vertical period to obtain a secondcounted value; and (g) determining if said first counted value isgreater than or equal to said minimum horizontal value and less than orequal to said maximum horizontal value; (h) determining if said secondcounted value is greater than or equal to said minimum vertical valueand less than or equal to said maximum vertical value; and (i)determining than an actual synchronous signal is contained in said inputimage signal if said first counted value is greater than or equal tosaid minimum horizontal value and less than or equal to said maximumhorizontal value and said second counted value is greater than or equalto said minimum vertical value and less than or equal to said maximumvertical value.